FIELD OF THE INVENTION
The invention relates to turbine casing of two dissimilar metals for elevated steam parameters application.
BACKGROUND OF THE INVENTION
In a typical larger power stations based on the steam parameters, the steam turbines are split into three separate modules, the first being the high pressure module, the second being the intermediate pressure module and the third being the low pressure module.
These turbines are designed to efficiently convert thermal energy into mechanical energy.
Today, it is a global quest to utilize fossil fuel more efficiently in power plants to achieve climate related goals on national as well as at a global front. To increase the efficiency of thermodynamic cycles of steam power plants the classical method is to increase the operating parameters such as input steam parameters. But higher the temperature of operation, lesser is the resistance of material to creep, a time-dependent failure of materials at elevated temperature under applied stresses. The utility of materials, exposed to elevated temperature and pressure, is limited by their capacity to sustain elevated temperatures without undergoing loss of strength over prolonged

exposure. Today, designers are working to utilizing these turbines which shall have input steam temperature to an extent higher than 700°C.
For such a temperature, steam turbine including cast components such as inner or admission casing are formed of exotic alloy materials apt to demonstrate desired mechanical properties at elevated temperature by withstanding such ever increasing operating conditions of more than 700°C, extent of difference in fluid pressure and temperature at inlet and exit of the turbine. Turbine casings often constitutes considerable material of the turbine and therefore significantly contributes to the cost of the turbine. If these casings incorporate high cost, high temperature metal alloys in entirety of their formation, the cost of the turbine module is even higher.
The manufacture of such large casings also demand the provisions of correspondingly large manufacturing facilities, with the associated lifting and manipulation means. This limits the supplier competition which thereby push the cost further up.
The present disclosure, a bimetallic turbine casing with its constructional features and its method of manufacturing is disclosed that includes a high temperature material section formed of a high temperature material, and a low temperature material section formed of a low temperature material. The low

temperature material section joined to an end of the high temperature material section.
SUMMARY OF THE INVENTION
The present invention proposes a bimetallic turbine casing with novel features in the casing which shall be capable to operate for a temperature range upto 710℃ and to meet the operational requirements. It uses a high temperature material (HTM), such as but not limited to nickel based alloys, on the high temperature sides and uses a low temperature material (LTM), such as but not limited to 12% chrome alloy steels on the intermediate to low temperature, pressure sides because of their superiority in sustainable mechanical properties in this regime. At the location where the temperature is high enough to mitigate the use of LTM a bimetallic joint, such as a weld joint, between a high cost HTM and a low cost LTM is envisaged to lower the product cost while the casing remains integrated with mechanical abilities under constant influence of service steam parameters it is exposed to.
OBJECT OF THE INVENTION
In accordance of the present invention, the primary object of the invention is to propose a bimetallic turbine casing with novel features that includes a high

temperature material section for high temperature application, and a low temperature material section for low temperature application.
An another object of the present invention is to propose a bimetallic turbine casing which will be capable of operative at elevated steam parameters with temperature and pressure range of up to 710°C and 300-350 kg/cm2 respectively.
Other advantages and features of the present invention will be better understood with reference to a preferred embodiment described hereinafter.
BRIEF DESCRIPTION OF THE ACCOMPANIED DRAWINGS
It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. Same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
Figure 1 illustrates the isometric sectional view of a bimetallic turbine casing.

Figure 2,3,4 illustrates the casing 10 arrangement with respect to turbine rotor 15 and outer or steam admission casing 31;
Figure 5,6 illustrates a method of preparing and using an cast article such as the static cast structure 10A, 10B.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
Although embodiments for the present subject matter have been described in language specific to package features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/device of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.
As a preferred embodiment, a welded high pressure turbine casing of two dissimilar metals for material and cost optimization is described in this application. It uses a HTM, such as but not limited to nickel based alloys, on the high temperature sides and uses a LTM, such as but not limited to 12% chrome alloy steels on the intermediate to low temperature, pressure sides.

Transition temperature section is the location where the temperature is low enough to mitigate the use of HTM and a bimetallic weld between a high cost HTM and a low cost LTM is envisaged to lower the product cost while the casing remains integrated with mechanical abilities under constant influence the service steam parameters they are exposed to.
Referring now to figure 1 in detail, a turbine casing is illustrated as a structure 10 in the high pressure turbine. Being in two halves, these casing join together at a joint plane 11 using plurality of threaded means 12 such as the studs and capnuts or the likes. This static structure 10 experiences a maximum service temperature of about 700-710°C and utilizes a combination of HTM 10A and LTM 10B as its material of construction for steam temperature zones above and below transition temperature respectively. Details for atleast one method of preparing and using an article such as the static cast structure 10A, 10B has been drawn in figure 5 in which separate molten masses 51 of HTM and LTM are casted 52 into respective molds that can impart critical design features in the cast structure 10 as required and detailed in description of figure 2, 3 and 4. The separate castings sections of HTM and LTM are thereafter optionally post-processed 56 as necessary. Post processing may include repairing cast weld defects 53, heat treatment 54 and required machining 55.

Figure 6 depicts an approach 62 for obtaining a dissimilar metal joint between the HTM and LTM cast structures 10A, 10B at transition temperature location 10C. The welding may be accomplished, by any operable approach 61, with or without a filler metal. Where used, the filler metal is preferably but not necessarily of the same composition as the piece 10A, 10B. After welding, the article is optionally post-processed 62 such as by heat treating or machining to obtain casing 10.
Figure 2 shows that the high pressure steam MS flows through two admissions into the casing 10 and into the first stationary blade row 13 with full-arc admission. A seal 40 is provided between the admission casing and the casing 10, so that the HP steam is not in contact with the admission casing 31 or a turbine outer casing, which shall result into smaller wall thicknesses in the admission casing.
The HP turbine section is equipped with two shaft seals 16, 23 to seal the turbine casing against outside at the point where the shaft penetrates the admission casing 31 and barrel casing exhaust section 33 at the front and rear.
Balance piston 17 is the integral part of turbine system, which is used to balance the residual thrust of the steam flow path. Number of seals 18 are provided in between the casing 10 and the balance piston 17. The difference in pressure upstream and downstream shall govern the sealing length.

Seal 19 seals the space between the casing 10 and the admission casing section against the space behind the piston 17, while allowing axial displacement between casing 10 and the admission casing.
When assembled, the casing 10 is supported on collar 30 in the admission casing 31 in the horizontal plane and is centered in the vertical plane via parallel keys, as shown in figure 3. This arrangement shall allow free thermal expansion of the casing 10 in all radial directions and from a fixed point in the axial direction. This ensures that the casing remains concentric with the turbine rotor 15. The axial fixed point for the casing 10 comprises a collar 30 on the casing which bears against a shoulder in the admission casing 31. Axial thrust acting on the casing 10 is transmitted to and accommodated by the threaded ring 22, 32 while the axial thermal expansion of the casing 10 shall originate at 30.
The welding location 10C shall be just after temperature suitable for weld joint during service. If the location of weld 10C falls just under the collar 30, it requires a larger weld thickness because of geometry of collar. In such cases, the location 10C may be offset towards the LTM 10B section. Shifting of weld location 10C also provide spatial clearance between 30 and welding means towards 33, thus facilitating improved weld quality due to unrestricted approach and smooth welding contour. High pressure turbine main steam inlet 14 sections profile around the periphery may be designed for keeping structure

10 stresses to suit material strength allowable under various operating conditions. The inlet section 14 shall be maintaining laminar steam mass distribution at first blade row.
The steam inlet arrangement 40 comprise of various assembly components mounted on the casing 10 to ensure smooth steam transition into the main steam inlet section 14. Mounting of these components needs a minimum wall thickness to hold the assembly.
As shown in figure 4, The steam injection is provided through storage chamber 21 for increasing power output by upto 5-7% beyond the rated output of the turbine. The storage chamber space within the casing 10 wall receives steam from a separate connection. The chamber used to provide extra steam ES into the main flowpath after desired stage as per the design requirement. Storage chamber constructed inside the casing wall radially for the desired angle in both the halves to ensure even steam distribution into the flow path. The length of storage chamber 21 may be finalized as per the operational requirements.
The optimum thicknesses for casing 10 be worked out based on the geometrical constraints and working parameters. In order to validate the casing 10 thickness, FEM techniques may be used for whole or selected sections.

Although the present discussion relates to bimetallic turbine casing, more particularly, the constructional features discussed hereinabove shall enable the casing to be operable in a temperature range of 700-710°C.
It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other cast structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various arrangements that, although not explicitly described or shown herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions.

WE CLAIM
1) A bimetallic turbine casing 10 capable of operating at temperature and pressure range of above 700-710°C and 300-350 kg/cm2 respectively and comprising of:
- high temperature material section 10A and low temperature material section 10B joined together at temperature transition point 10C;
- seal 40 between the admission casing and the casing 10 to avoid high temperature high pressure steam contact with admission and to ensure smooth steam transition into the main steam inlet section 14;
- seals 18 provided in between the casing 10 and the balance piston 17.
- seal 19 to seal the space between the casing 10 and the admission casing section against the space behind the piston 17, while allowing axial displacement between casing 10 and the admission casing.
- threaded ring 22 to accommodate axial thrust acting on the casing 10;
- shaft seals 16, 23 to seal the turbine casing against outside at the point where the shaft penetrates the admission casing 31 and barrel casing exhaust section 33 at the front and rear end;
- storage chamber 21 for increasing power output beyond the rated output of the turbine;
- to provide extra steam ES into the main flowpath after desired stage as per the design requirement.

2) The bimetallic turbine casing as claimed in claim 1, wherein while being assembled, the casing 10 is supported on collar 30 in the Admission casing 31 in the horizontal plane and is centered in the vertical plane via parallel keys to allow free thermal expansion of the casing 10 in all radial directions and from a fixed point 30 in the axial direction.
3) The bimetallic turbine casing as claimed in claim 1, wherein the welding location 10C shall be just after transition temperature suitable for weld joint during service or at an offset position, if required, towards the LTM 10B section.
4) The bimetallic turbine casing as claimed in claim 1, wherein the high pressure turbine main steam inlet sections profile may be designed for keeping structure 10 stresses to suit material strength allowable under various operating conditions.
5) The bimetallic turbine casing as claimed in claim 1, wherein the optimum thicknesses for casing 10 be worked out based on the construction features and manufacturing feasibility suiting working parameters.

6) The bimetallic turbine casing as claimed in claim 1, wherein the difference in pressure upstream and downstream shall govern the sealing 18 length.